Shape memory materials have been known for over half a century. Thermoresponsive shape memory polymers (SMP's), in particular, have attracted great interest due to their large strain recovery capability (up to 800%), much greater than what is achievable with shape memory alloys (SMA's) with maximum strain recovery below 8%. Besides the large strain recovery ability, SMP's hold other advantages over SMA's including light weight, excellent processability, and low cost.
For a polymer to possess shape memory properties, it has to have a frozen phase and a reversible phase. The former is responsible for memorizing the original shape and is usually achieved via chemical cross-linking or physical cross-linking (e.g., chain entanglement and crystallization). The fixing and change of temporary shapes, on the other hand, are due to the reversible phase, which can be either a glass transition or a melting transition. Since most polymers possess a glass transition temperature (Tg) or a melting temperature (Tm) or both, they can be converted into SMP's by introducing a freezing mechanism (e.g. chemical cross-linking). The ample opportunities to create and tailor the shape memory properties of SMP's through molecular design are exemplified by the variety of SMP's discovered and the wide range of Tg (from −30 to +70° C.) obtainable with polyurethane SMP's.
Owing to their superior mechanical properties, ease of processing, excellent chemical resistance, and good adhesion to a wide range of substrates, epoxy based polymers have been widely used as structure, coating, and adhesive materials in real world applications.